Such a method and such a circuit arrangement are known, the latter being illustrated schematically in FIG. 1. On the input side, the supply voltage Ue, for example the system voltage, is present at a dimming device 10 that has an input 8 for a dimming selection and preferably includes a triac or a thyristor. Following thereupon is a filter device 12 that contains two independent filter inductors L1, L2 and two capacitors C1, C2. In this case, it also has a rectifier circuit 14 that comprises the diodes D1 to D4. Following thereupon is a step-up converter 16 that comprises an inductor L3, a diode D5, a capacitor C3, a resistor R2 and a switch S1. Provided at the output of the step-up converter 16 is the so-called intermediate circuit voltage Uzw that is coupled to the input of an electronic ballast. For the following reasons, the embodiment illustrated in FIG. 1 is suitable only for a specific system voltage, for example 120 V or 277 V, depending on the dimensioning of the filter device 12. Specifically, if the dimming device 10 is fitted with a triac or a thyristor, the input impedance of the downstream filter device 12 is to behave in a fashion similar to a resistive load. If the load is not resistive, undesired effects occur or the circuit arrangement no longer functions at all. The optimum values for the inductors L1 and L2, as well as for the capacitors C1 and C2 of the filter device 12 differ in the case of dimensioning for 120 V from those in the case of dimensioning for 277 V.
FIG. 2 shows the time profile of the current IL1 through the inductor L1, of the voltage U1 dropping at the output of the dimming device 10, and the voltage UC2, which drops at the capacitor C2, in each case for operation of a circuit arrangement dimensioned to 120 V in accordance with FIG. 1, and operation with 120 V. Disregarding small overshoots, FIG. 2 shows the optimum profile of these variables. If the circuit arrangement according to FIG. 1 and dimensioned to 120 V is now operated with 277 V, short large current peaks are produced at the rising edge of the input voltage—see FIG. 3. The behavior of the filter device 12 is then capacitive and an interruption of the input current already occurs after a short time, approximately 100 to 300 μs, since said input current vanishes because of the oscillation behavior. As a result, the current IL1 sinks below the holding current threshold of the triac or of the thyristor in the dimming device 10, and the dimming device 10 switches off.
FIG. 4 shows the time profile of the variables IL1, U1, UC2 in the case of dimensioning of the circuit arrangement in accordance with FIG. 1 for 277 V and operated with an input voltage Ue of 120 V. A high-frequency oscillation is set up that likewise leads to a result of no use.